The present invention relates to imageable lithographic plates for printing.
Plates of interest have a solvent-soluble, radiation-polymerizable, oleophilic resin coating on a hydrophilic substrate. In conventional practice, after image-wise exposure at ultraviolet (UV), visible, or infrared (IR) wavelengths, the plates are developed with solvent to remove the unexposed areas of the coating by dissolution, thereby producing a substantially planographic pattern of oleophilic and hydrophilic areas. The developed plates are then ready for mounting on a cylinder of a printing press, where the plates are subjected to fountain fluid and ink for transfer of ink to a target surface according to the pattern of oleophilic and hydrophilic areas on the plate.
The imaging radiation produces a cross-linking reaction in the imaged areas, which increases the mechanical adhesion of the image areas to the grained surface of the substrate, and also increases the cohesion (hardening) of the image area so that it can withstand the abrasive effect of receiving and transferring ink during the production run on-press. It is known that the hardness of the image areas can be increased by preheating the plate (i.e., before exposure to imaging radiation), but this must be done at a relatively low temperature such as 105° C., for a relatively short period of time (such as one minute) because the entire coating experiences a modest degree of cross-linking. If the temperature and/or time of preheat exceed a relatively low threshold, the cross-linking of the entire coating will degrade the chemical processing, which will not completely remove all of the non-image areas from the substrate. On the other hand, the less the intensity of the preheat, the more likely some of the material in the image areas will not cross-link and therefore dissolve during the development step.
Regardless of how manufacturers and end users make this tradeoff, in conventional solvent based development of negative, actinically imageable lithographic plates, no further cross-linking can be achieved in the image areas after development of the plate in the solvent. Any coating material in the image areas that did not react with the radiation, is dissolved and therefore removed from the image areas during the development step.
With development using only mechanical forces, as described in co-pending U.S. application Ser. No. 12/799,568 and Ser. No. 12/586,764 (the disclosures of which are hereby incorporated by reference), preheating was disclosed as a desirable option, but also performed at a modest level of intensity so as not to cross-link the unimaged areas to the extent that the mechanical forces cannot remove the nonimage areas during development. The plates are heated after imaging but before development to increase the difference in cohesion and adhesion of the coating to the substrate as between the imaged and unimaged areas, such that a greater force can be applied to the plates to dislodge only the unimaged areas. In particular, a thermally imageable negative working plate can be exposed to heat for a short period of time after imaging, whereby the imaged portions become more stable and tougher, while the portions of the coating that are to be removed are not significantly affected. The heating step preferably, but not necessarily, immediately follows the imaging step, but can be at a different location from the imaging step. The mechanical development of imaged plates has numerous advantages over known techniques that rely on solubilization or dispersion for removal of the unimaged areas. These advantages include retention of the full integrity of the imaged areas, avoiding the handling of chemical waste product, and the option of developing either prepress or on-press. Moreover, with on-press mechanical development, neither the ink nor fountain solution is contaminated by solubilized material that can affect the quality of the printing.